This invention relates to a method of and apparatus for modifying a video signal and, more particularly, to a method and apparatus wherein the vertical blanking interval of the video signal is modified such that a video picture can be reproduced directly from the modified video signal but an accurate video picture cannot be reproduced if the modified video signal is recorded and subsequently played back on, for example, a video signal recorder.
In some instances, it is desirable to modify a video signal so as to prevent unauthorized use thereof. For example, with the recent introduction of video signal recording systems, such as video tape recorders (VTR), it may be useful to discourage unauthorized recording of broadcasted television programs by modifying the video signals in a manner which would prevent the reproduction of a video picture in response to the recording and playing back of such video signals, but which would permit a video picture to be reproduced directly from the reception of the broadcasted video signals. As another example, if master video recording media, such as video tapes, are produced containing television broadcasting, it may be advantageous to modify the signals recorded on such tapes so that they can be played back in order to reproduce a video picture directly, but which would prevent the reproduction of a video picture if such tapes are copied.
Although such video signal modification can be attained by using unique encoding and decoding apparatus, such unique apparatus is quite expensive, especially since each television receiver would have to be provided with the decoding apparatus in order to reproduce a video picture from a received, encoded video signal.
One proposal for modifying a video signal in order to avoid the so-called piracy thereof is described in U.S. Pat. No. 3,963,865. This proposal proceeds upon the recognition that various VTR systems which are used, for example, in home entertainment systems, must detect particular signals included in the periodic vertical blanking interval of the video signal in order to achieve accurate reproduction of a video picture from the recorded video signals. In particular, such VTR systems detect the vertical synchronizing pulses included in the vertical blanking interval for the purpose of generating control signals having a frequency equal to the vertical blanking frequency. Control pulses are derived from these control signals and are recorded on a separate control track of the video recording tape by the VTR. During playback, the recorded control pulses are used to synchronize the rotation of the rotary playback heads with respect to the movement of video tape. If these control pulses are not recorded, the necessary synchronization between the playback heads and tape movement is lost, thereby resulting in a distorted, unintelligible video picture. In some VTR systems, the reproduced video signals are not transmitted to a television receiver, such as a television monitor, if the aforementioned control pulses are not detected.
In VTR systems of the aforedescribed type, the vertical synchronizing pulses included in a received video signal are detected by integrating all of the pulses contained in the vertical blanking interval. Because of the particular wave shape of the vertical synchronizing pulses, as established by, for example, NTSC standards, the integrated output will exceed some threshold level after a particular number of vertical synchronizing pulses has been received. A similar detection process is used in a conventional television receiver, except that a smaller number of vertical synchronizing pulses need be integrated for detection.
In accordance with these factors, the proposal described in U.S. Pat. No. 3,963,865 modifies the vertical blanking interval of a video signal by effectively "removing" all but one and one-half of the vertical synchronizing pulses included in the blanking interval. According to this patent, the integration of one and one-half vertical synchronizing pulses is sufficient for the proper operation of a television receiver but more vertical synchronizing pulses must be integrated for the recording of control pulses by a VTR system. In one embodiment, the removed vertical synchronizing pulses are replaced merely by a constant DC level. In another embodiment, the DC level of the removed vertical synchronizing pulses is shifted such that these pulses are not detected, and thus not integrated, either by the television receiver or by the VTR circuitry. In still another embodiment, the DC level of both the removed vertical synchronizing pulses and the equalizing pulses which normally follow the vertical synchronizing pulses is shifted to prevent such pulses from being detected.
By removing, or effectively removing, a substantial portion of the pulses included in the vertical blanking interval, horizontal synchronizing problems in a television receiver may result. That is, the automatic frequency control (AFC) circuit which is used in the television receiver to lock the horizontal deflection circuitry to the horizontal synchronizing pulse frequency cannot respond to the removed pulses during the vertical blanking interval. As is conventional, the vertical blanking interval includes one set of equalizing pulses of twice the horizontal line rate, followed by a set of vertical synchronizing pulses also of twice the horizontal line rate, followed by another set of equalizing pulses. The AFC circuit in the television receiver utilizes these equalizing and vertical synchronizing pulses during the vertical blanking interval to maintain a locked, or synchronized relation of the horizontal deflection circuitry. However, if a large portion of the vertical synchronizing pulses is removed, this locking relation cannot be maintained. Consequently, the first few lines of the video picture displayed by the television receiver may be distorted until the AFC circuit once again is synchronized to the horizontal line interval of the received video signal.